1. Field of the Invention
The present invention relates to a plural terminals communication controlling system which accommodates a plurality of communication terminals into a single communication line by using an additional device having one central office line terminal connected to the communication line and a plurality of communication terminal connecting terminals connected to communication terminals. This system is capable of connecting with a predetermined communication terminal even if a transmission side has no means for transmitting a selection signal specifying a communication terminal, and is also capable of connecting with the other predetermined communication terminal even if this predetermined communication terminal does not respond or is an automatically answering telephone set.
2. Description of the Prior Art
FIG. 1 shows a plural terminals communication controlling system of a conventional type which calls a communication terminal specified by the transmission side by connecting a single telephone line with plural communication terminals. In FIG. 1, reference numeral 1 represents a telephone exchanger; 2, a plural terminals communication controlling system; 3a and 3b, telephone sets; 4a and 4b, facsimiles; 5a and 5b, data terminals; l1 and l2, telephone lines; T1, a telephone line connecting terminal and T2 (T2a-T2c), communication terminal connecting terminals. This plural terminals communication controlling system 2 comprises a ringing signal detecting circuit 22a, a loop closing circuit 22b, a selection signal receiving circuit 22c, a connecting switch changing circuit 23, connection switches 26 (26a-26c), a ringing signal transmitting circuit 27, response detecting circuits 29 (29a-29c), and a control circuit 21.
This system is so constructed that a ringing signal is transmitted from the telephone exchanger 1 in response to a signal originating from the data terminal 5b. The ringing signal detecting circuit 22a detects a ringing signal. Then, the control circuit 21 receives a signal from the ringing signal detecting circuit 22a and actuates the loop closing circuit 22b to establish a communication line between the plural terminals communication controlling system 2 and the data terminal 5b. Next, the selection signal receiving circuit 22c detects a selection signal from the data terminal 5b for connecting with the data terminal 5a coupled with the communication terminal connecting terminal T2c, and in response to the selection signal, the control circuit 21 closes the connection switch 26c to the side of the ringing signal transmitting circuit 27. The ringing signal transmitting circuit 27 is operated to transmit the ringing signal. Then, when the response detecting circuit 29c detects a response from the data terminal 5a responding to this ringing signal, the transmission of the ringing signal is stopped by stopping the operation of the ringing signal transmitting circuit 27 and the connecting switch 26c is closed to the side of the telephone line l1 so that communication between the data terminal 5a and the data terminal 5b is established.
Normally, the purpose of a call incoming to the above system made by an ordinary telephone subscriber is a "telephone conversation". In this case, most likely, the telephone subscriber has no knowledge about the transmitting method of the selection signal for calling the telephone set and therefore, no selection signal is received by the above system. In order to prevent deterioration of the service to the ordinary telephone subscribers, the control circuit 21 supervises whether or not the selection signal specifying a communication terminal is detected within a predetermined period of time and calls the telephone set 3a when the selection signal is not detected.
In such construction, the telephone set is called when the selection signal is not detected within the predetermined period of time. That is, in the case where a terminal on the transmission side is for example, a facsimile machine having no selection signal transmitting circuit, though it is required to call the facsimile connected with the additional device, the selection signal specifying a communication terminal is not detected by the selection signal receiving circuit and, therefore, the telephone set is called. This creates a problem of the incapability of communication with the facsimile.
Like a facsimile machine, this problem is extremely important in the case where the transmission side has no selection signal transmitting circuit for specifying a communication terminal.